This invention relates to electrical cable terminals, and more particularly to terminals for high speed cables with multiple conductors.
Certain high speed electronic cable terminals employ arrays of spring pins to contact pads or lands of a circuit board or integrated circuit under test, or to contact connections of an electronic device for a permanent connection. The spring pins are straight, elongated pins received in cylindrical sleeves, and which are axially biased by spring pressure to an extended position. All pins extend in the same direction, with all the pin tips in a common plane. Contact is made by aligning the terminal with the device being probed or contacted, and applying axial pressure to ensure contact by each pin with a minimum pressure. The range of motion of each pin accommodates contour variations in the device being contacted, and slight variations in the position of each pin.
For applications in which very high frequency signals are to be transmitted, the cable to which the terminal is connected may be formed of coaxial wires, each shielded to provide consistent performance and to prevent cross talk and other electronic interference. Where the cable joins the terminal, and within the terminal, discontinuities may occur that limit performance. To minimize these discontinuities, existing terminals employ special pin assemblies that essentially preserve the coaxial nature of the cable. Each pin in such assemblies is received in a thin inner conductive sleeve within an insulating cylindrical body that is sized to match the dielectric layer that surrounds the central conductor of the cable wire. A metal sleeve surrounds the body, analogous to the cable wire shield. Hereafter, these assemblies referred to as matched impedance spring pins. To join the cable to the pin assembly, the cable""s central conductor is soldered to the inner sleeve, and an outer junction sleeve is soldered to the cable shield and the assembly""s outer sleeve, spanning the gap to provide an effectively continuous shield to the point where the pin protrudes from the body. To align several of these assemblies to form an array, a conductive block having an array of bores is used. Each bore is precisely sized to receive the outer sleeve of each assembly. Bore size and sleeve diameter must be closely controlled with very limited tolerance for variation. If the fit is too loose, there will be limited points of contact, and the grounding and shielding effect of the block will be non-uniform. If the fit is too tight, the dielectric body of the pin assembly may be compressed, changing the dielectric gap, and altering performance characteristics. The matched impedance spring pins must also be precisely manufactured to maintain consistent diameters. Manufacturing tolerances result in a minimum number of interference points between the matched impedance spring pin outside diameter and the conductive block inside diameters. These minimal contacts result in ground return path inconsistencies reducing the electrical signal integrity.
To provide these close tolerances requires manufacturing techniques that substantially increase component part cost. In addition, the assembly time and cost is significantly high, because each pin assembly must be separately connected to the cable, so that labor costs increase with the number of pins employed.
In addition, the pin assemblies have outer diameters determined by the dielectric thickness needed to match that of the cable wires. This diameter limits the spacing of the pin contact points, preventing spacings less than the outer sleeve diameter. In some applications, especially where many contacts are required, or where test pads are required to occupy valuable space on a printed circuit wafer or board, this necessarily wide spacing increases costs.
The present invention overcomes the limitations of the prior art by providing a cable assembly. The cable assembly includes a printed circuit board having a pattern of conductive traces on a major surface of the board. The cable has a number of conductors electrically connected to the traces, and a number of elongated spring contact elements are connected to the board. Each contact element has a sleeve, known as a receptacle, oriented parallel to the major surface and connected to a first pad portion of a selected trace. Each contact element has an axially movable pin, known as a spring pin, slidably received in the receptacle and extending beyond a first peripheral edge portion of the board. The position of the electrical elements of the design are located to simulate the performance of a matched impedance spring pin assembly.